add related_info on page-replacement-policy algorithms
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related_info/lab3/page-replacement-policy.md
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related_info/lab3/page-replacement-policy.md
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# 理解页置换算法
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## 选择四种替换算法（0：LRU置换算法，1:改进的clock 页置换算法，2：工作集页置换算法，3：缺页率置换算法）中的一种来设计一个应用程序（可基于python, ruby, C, C++，LISP等）模拟实现，并给出测试。请参考page-replacement-policy.py 代码或独自实现。
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###page-replacement-policy.py 代码
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#### 对模拟环境的抽象
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虚拟页访问序列：addresses=1,2,3,4,0,5.... 这里面的最大值代表最大页号
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页替换算法：policy=FIFO, LRU, OPT, CLOCK
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CLOCK算法用的bit位数：clockbits=1
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物理页帧大小：cachesize
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实际保持的也访问序列：addrList [1,2,3,4,0,5,...]
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物理页帧内容：memory [] 初始为空
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当前占用的页帧数量：count 初始位0
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#### 执行过程描述
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```
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for nStr in addrList:
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# for clock need to track the ref by reference bits
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try:
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idx = memory.index(n)
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hits = hits + 1
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if policy == 'LRU' :
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....
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except:
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idx = -1 #missing
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miss = miss + 1
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if idx=-1 and ...: #missing and need replacement
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#if phy page frames are full
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# for FIFO , LRU
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# replace victim item from memory by " victim = memory.pop(0)"
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# for CLOCK
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# find one page for the beginning of scan
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# check ref[page] and update ref[page]
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# find a victim which ref[page]=0 by memory.remove(page)
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else:
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# miss, but no replacement needed (phy page frame not full)
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# now add to memory
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#update ref for clock replacement
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```
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related_info/lab3/page-replacement-policy.py
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related_info/lab3/page-replacement-policy.py
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#! /usr/bin/env python
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import sys
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from optparse import OptionParser
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import random
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import math
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def hfunc(index):
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if index == -1:
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return 'MISS'
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else:
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return 'HIT '
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def vfunc(victim):
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if victim == -1:
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return '-'
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else:
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return str(victim)
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#
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# main program
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#
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parser = OptionParser()
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parser.add_option('-a', '--addresses', default='-1', help='a set of comma-separated pages to access; -1 means randomly generate', action='store', type='string', dest='addresses')
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parser.add_option('-p', '--policy', default='FIFO', help='replacement policy: FIFO, LRU, OPT, CLOCK', action='store', type='string', dest='policy')
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parser.add_option('-b', '--clockbits', default=1, help='for CLOCK policy, how many clock bits to use', action='store', type='int', dest='clockbits')
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parser.add_option('-f', '--pageframesize', default='3', help='size of the physical page frame, in pages', action='store', type='string', dest='pageframesize')
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parser.add_option('-s', '--seed', default='0', help='random number seed', action='store', type='string', dest='seed')
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parser.add_option('-N', '--notrace', default=False, help='do not print out a detailed trace', action='store_true', dest='notrace')
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parser.add_option('-c', '--compute', default=False, help='compute answers for me', action='store_true', dest='solve')
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(options, args) = parser.parse_args()
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print 'ARG addresses', options.addresses
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print 'ARG policy', options.policy
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print 'ARG clockbits', options.clockbits
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print 'ARG pageframesize', options.pageframesize
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print 'ARG seed', options.seed
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print 'ARG notrace', options.notrace
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print ''
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addresses = str(options.addresses)
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pageframesize = int(options.pageframesize)
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seed = int(options.seed)
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policy = str(options.policy)
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notrace = options.notrace
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clockbits = int(options.clockbits)
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random.seed(seed)
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addrList = []
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addrList = addresses.split(',')
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if options.solve == False:
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print 'Assuming a replacement policy of %s, and a physical page frame of size %d pages,' % (policy, pageframesize)
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print 'figure out whether each of the following page references hit or miss'
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for n in addrList:
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print 'Access: %d Hit/Miss? State of Memory?' % int(n)
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print ''
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else:
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if notrace == False:
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print 'Solving...\n'
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# init memory structure
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count = 0
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memory = []
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hits = 0
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miss = 0
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if policy == 'FIFO':
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leftStr = 'FirstIn'
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riteStr = 'Lastin '
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elif policy == 'LRU':
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leftStr = 'LRU'
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riteStr = 'MRU'
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elif policy == 'OPT' or policy == 'CLOCK':
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leftStr = 'Left '
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riteStr = 'Right'
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else:
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print 'Policy %s is not yet implemented' % policy
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exit(1)
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# track reference bits for clock
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ref = {}
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cdebug = False
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# need to generate addresses
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addrIndex = 0
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for nStr in addrList:
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# first, lookup
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n = int(nStr)
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try:
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idx = memory.index(n)
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hits = hits + 1
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if policy == 'LRU' :
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update = memory.remove(n)
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memory.append(n) # puts it on MRU side
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except:
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idx = -1
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miss = miss + 1
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victim = -1
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if idx == -1:
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# miss, replace?
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# print 'BUG count, pageframesize:', count, pageframesize
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if count == pageframesize:
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# must replace
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if policy == 'FIFO' or policy == 'LRU':
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victim = memory.pop(0)
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elif policy == 'CLOCK':
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if cdebug:
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print 'REFERENCE TO PAGE', n
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print 'MEMORY ', memory
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print 'REF (b)', ref
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# hack: for now, do random
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# victim = memory.pop(int(random.random() * count))
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victim = -1
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while victim == -1:
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page = memory[int(random.random() * count)]
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if cdebug:
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print ' scan page:', page, ref[page]
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if ref[page] >= 1:
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ref[page] -= 1
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else:
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# this is our victim
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victim = page
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memory.remove(page)
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break
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# remove old page's ref count
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if page in memory:
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assert('BROKEN')
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del ref[victim]
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if cdebug:
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print 'VICTIM', page
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print 'LEN', len(memory)
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print 'MEM', memory
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print 'REF (a)', ref
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elif policy == 'OPT':
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maxReplace = -1
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replaceIdx = -1
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replacePage = -1
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# print 'OPT: access %d, memory %s' % (n, memory)
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# print 'OPT: replace from FUTURE (%s)' % addrList[addrIndex+1:]
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for pageIndex in range(0,count):
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page = memory[pageIndex]
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# now, have page 'page' at index 'pageIndex' in memory
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whenReferenced = len(addrList)
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# whenReferenced tells us when, in the future, this was referenced
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for futureIdx in range(addrIndex+1,len(addrList)):
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futurePage = int(addrList[futureIdx])
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if page == futurePage:
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whenReferenced = futureIdx
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break
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# print 'OPT: page %d is referenced at %d' % (page, whenReferenced)
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if whenReferenced >= maxReplace:
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# print 'OPT: ??? updating maxReplace (%d %d %d)' % (replaceIdx, replacePage, maxReplace)
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replaceIdx = pageIndex
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replacePage = page
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maxReplace = whenReferenced
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# print 'OPT: --> updating maxReplace (%d %d %d)' % (replaceIdx, replacePage, maxReplace)
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victim = memory.pop(replaceIdx)
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# print 'OPT: replacing page %d (idx:%d) because I saw it in future at %d' % (victim, replaceIdx, whenReferenced)
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else:
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# miss, but no replacement needed (page frame not full)
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victim = -1
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count = count + 1
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# now add to memory
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memory.append(n)
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if cdebug:
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print 'LEN (a)', len(memory)
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if victim != -1:
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assert(victim not in memory)
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# after miss processing, update reference bit
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if n not in ref:
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ref[n] = 1
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else:
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ref[n] += 1
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if ref[n] > clockbits:
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ref[n] = clockbits
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if cdebug:
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print 'REF (a)', ref
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if notrace == False:
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print 'Access: %d %s %s -> %12s <- %s Replaced:%s [Hits:%d Misses:%d]' % (n, hfunc(idx), leftStr, memory, riteStr, vfunc(victim), hits, miss)
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addrIndex = addrIndex + 1
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print ''
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print 'FINALSTATS hits %d misses %d hitrate %.2f' % (hits, miss, (100.0*float(hits))/(float(hits)+float(miss)))
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print ''
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